As the integrated circuit fabrication industry continually moves to improve yield, there is a need to provide faster feedback so as to catch any drifts in the process as early as possible. Thus, there has been much activity in the field of integrated circuit metrology. As the term is used herein, “integrated circuit” includes devices such as those formed on monolithic semiconducting substrates, such as those formed of group IV materials like silicon or germanium, or group III-V compounds like gallium arsenide, or mixtures of such materials. The term includes all types of devices formed, such as memory and logic, and all designs of such devices, such as MOS and bipolar. The term also comprehends applications such as flat panel displays, solar cells, and charge coupled devices.
Ellipsometry has been used extensively in integrated circuit metrology. To meet the desired precision for complicated film stacks, ellipsometry has generally been implemented with either multi wavelength or multi angle approaches. A wide wavelength range is generally desirable for spectroscopic ellipsometry. However, chromatic aberrations tend to create serious challenges when attempting to focus the beam to a small spot on the substrate, and then process all of the wavelengths in the desired range at the same time. Thus, these two approaches are typically enabled with the use of one or more rotating elements within the ellipsometer. Therefore, ellipsometers having rotating elements are one current method of implementing a multi wavelength solution.
High performance rotation-based spectroscopic ellipsometers employed in integrated circuit fabrication units tend to fall into one of two generally classes of instruments. The first class is generally referred to as a rotating polarizer, and the second class is generally referred to as a rotating analyzer or compensator. Because of the difficulty involved in integrating these two types of ellipsometers into process modules that are used for inline integrated circuit inspections, a reflectometer is generally used for such measurements, instead of an ellipsometer.
The use of a motor in the ellipsometer, to provide the rotation capability, has been one of the biggest reliability problems, and adds greatly to the cost of the ellipsometer. The motor should be controlled very accurately in order to produce accurate and repeatable measurements. Therefore, large encoders have been used with the motors to ensure accurate placement of the rotating elements, and the rotation of the elements is typically limited to a speed that is on the order of a few Hertz. In addition, lamp noise and mechanical stability have been a major source of system error in ellipsometers. Therefore, these systems tend to be stand alone units, where the reliability requirement is less stringent than in an inline process module.
Another method to implement a multi wavelength solution is to use a focusing element based on all reflective optics to obtain the ellipsometric parameters. Based on such reflective optics, an ultra-wide spectrum of light, from vacuum ultra violet to infrared, can be collimated and focused onto a very small spot for spectroscopic ellipsometric measurements.
Yet another approach is disclosed in U.S. Pat. No. 6,275,291, the disclosure of which is included by reference herein as if laid out in its entirety, and uses sub wavelength-spaced grooves that are aligned along different orientations. As disclosed in the latter-referenced patent above, FIG. 11 depicts a wave plate 21 mounted onto a detector 26, to form a combined structure 15. Two pixels 23 of the wave plate 21 are shown. The pixels 23 are formed by two grids, the major axis orientations 25 of which are at different angles. The grid structures 23 are preferably made of a transparent material, and form a unit with the transparent retarder substrate 19. Typical dimensions of such grid structures 23 are: width 200 millimeters, trench width 200 microns, depth 400 microns. When the light 12 falls on these structures, the surface reacts like an artificially generated anisotropic material, similar to the anisotropic crystals used in conventional optics for the manufacture of retarders.
These grids of the pixels 23 can be manufactured by means of electron beam lithography in combination with ion beam etching processes. After the manufacture of the grid, the grid substrate unit is preferably attached, such as by an adhesive, to the analyzer 5, and the latter is in turn connected to the detector 26, which preferably has a substrate 9 on which the charge coupled device pixels are located. The position of the axis of the analyzer 5 is preferably selected so that it does not coincide with one of the major axes 25 of the retarder 23. This becomes possible if the angles of the major axes 25 within a pixel group vary by about thirty degrees, while the axis of the analyzer 5 is at about forty-five degrees. The groove pitch of the pixels 23 is preferably less than the wavelength of the light 12. The orientation of the light 12 coming off of the grooves in the pixels 23 tends to change across the surface of the detector 26, acting like a rotating compensator. Therefore, no moving parts are needed for this approach.
However, there are a number of process related issues with this approach as describe in the latter-referenced patent. For example, the pixel 23 on the micro wave plate 15 and the charge coupled device 26 must line up. Further, electron beam and ion etch processes are needed to fabricate the wave plate. Variations in these processes tend to give inconsistent retardation and orientation across the charge coupled device, and the lithography processes used are not good at writing slanted lines at arbitrary orientation. The variations in the micro wave plate create problems with system-to-system matching. In addition, a large number of unknowns need to be calibrated. The unknowns determined through calibration tend to make it an inaccurate measurement system.
As introduced above, although reflectometers are typically used for integrated critical dimension and film metrology for the reasons given above, it is well known that ellipsometry is much more sensitive than reflectometry, since ellipsometry measures the change in polarization states, and is much less sensitive to lamp noise and transmission loss over time.
What is needed, therefore, is an ellipsometer that reduces, at least in part, some of the problems described above.